Since 9-11 in U.S., security-inspection of aviation is becoming more and more emphasized. Besides conventional security-inspection of packages, security-inspection of the liquid articles carried by passengers is added. Accordingly, it is in dire need of means and methods for quick security-inspection of the liquid articles.
Nowadays, there are four types of methods used in security-inspection of liquid articles, including chemical method, electromagnetic method, neutron detection method and radiation detection method as follows:
1) The chemical method can be subdivided into odor identification method, ion scanning explosive detection method and substance analysis method. The odor identification in practical applications often fails to implement detection because of sealed and packaged conditions of liquid articles. The ion scanning explosive detection method is known for its high sensitivity, but with high false alarm rate, it suffers from the affection of background environment. The substance analysis is of high precision and high accuracy, but this method needs a certain period of time to analyze the sample, which cannot meet the demands of on-site quick detection.
2) The electromagnetic method works in an active measurement manner. It distinguishes liquid articles from each other according to their dielectric constants in an electromagnetic field. The electromagnetic method is easily subjected to severe affection of metal packages or other thick material packages. As a result, the use of electromagnetic method is limited in case of complex package materials.
3) The use of the neutron detection method will leave residual radiation remaining in the detected liquid due to the effect of “neutron activation”. Furthermore, the radiation shielding is complicate due to neutrons' strong penetrability, and the apparatus has to take a large area, so the method is not suitable for application in the security-inspection systems of civil aviation.
4) Currently, most of the security-inspection apparatuses for civil aviation are radiation apparatuses. In these apparatuses, 2D X-ray imaging technology and three-dimensional CT technology are mostly adopted. These technologies, which are mainly used for security-inspection of packages, fail to accomplish the security-inspection of liquid articles.
The 2D X-ray imaging technology acquires two-dimensional projection images by integrating three-dimensional information of articles along X-ray. These images show internal structures of object in the form of grayscale or pseudo-color, to give the operator a vivid display. However, the 2D X-ray imaging technology is in defect of one-dimensional information of object, so the inspection of liquid articles is suffering from severe affection of shapes and sizes of the liquid articles.
The three-dimensional CT technology is the extension and application of CT technology. The CT technology was applied in the diagnostics first, which was implemented by conducting a multi-angle projection of respective slices of an article. By using a computer to reconstruct the multi-angle projection data of respective slices, reconstructed images were obtained. The information of the different attenuation coefficients in the reconstructed images was displayed in the form of different grayscales, by which the internal structures of the articles were displayed. With the development of the CT technology, the industrial CT for the nondestructive inspection and the package CT for the security-inspection were put into use, with the goal remaining to be acquiring the slice images of the internal structures of articles. It is dependent on human operator's analysis on the structure to determine whether the article is dangerous, and thus is not applicable in automatic and quick inspection. Furthermore, it is difficult for the security-inspection devices of the CT type to gain popularity, for their high cost and bulkiness, as a result of the wide range of the articles that can be detected by them.
To sum up, for the quick detection of the liquid articles, the chemical method, the electromagnetic method and the neutron detection method are not suitable. By using the 2D X-ray imaging technology and the three-dimensional CT technology, images showing the internal structure of the article are acquired, but these images cannot work as sufficient evidence for the automatic security inspection of the liquid articles.